A variety of vehicles have been designed for travel over snow, with the well known snowmobile type being by far the most prevalent and commercially successful. Snowmobiles have evolved into highly refined and capable vehicles for travel over snow and are used predominantly for recreation. Yet, these commercially available snowmobiles exhibit a number of undesirable features and characteristics that limit their agility and versatility. They are generally quite heavy, weighing typically from 400 to 600 pounds, and require powerful motors to provide power-to-weight ratios sufficient for high performance in highly variable power-sapping snow conditions and for the wide furrows they plow through the snow. A snowmobile typically has two outboard steering skis and a relatively wide, flat track. The outrigger positioning of the skis and the wide flat track constrain the snowmobile to travel flat upon the snow and inhibit banked or leaned turning. These features also tend to limit the steepness of slope that the snowmobile can traverse, since the snowmobile can easily slide sideways when traveling across a steep slope. When cornering on hard packed snow, a snowmobile cannot be easily banked to achieve tight-radius cornering, and the operator must move his or her body far to the inside of the turn to avoid tipping or side slipping the snowmobile.
A second much less common type of snow vehicle is the snow bike or snow cycle. These vehicles are generally smaller and lighter than snowmobiles. Snow cycle designs are usually based upon off-road motorcycles or scooters with the front wheel replaced by a ski and the rear wheel replaced by an endless loop traction belt, commonly called a track. Snow cycles are vehicles designed for banked turning like a motorcycle or bicycle, and are operated "in balance". Snow cycles typically have a single steering ski and a relatively narrow track located behind and in line with the single ski. In turns, a snow cycle is leaned sideways in the direction of the turn such that the sum of gravitational and centrifugal forces acting on the vehicle and operator bears centrally on the contact areas between the snow and the ski and between the snow and the track. In this way, the snow cycle and rider remain in balance through the turn. When traversing slopes, a snow cycle maintains a vertical position such that the gravitational forces acting on the snow cycle and operator intersect the approximate center of the contact areas between the snow cycle ski and the snow and between the track and the snow.
Motor-driven snow cycles are described in U.S. Pat. Nos. 5,474,146, 4,613,006 and 4,442,913. None of these snow cycles have achieved notable commercial success. The snow cycles described in the '006 and '913 patents have minimal suspension travel between the ski and the vehicle main frame and between the track support and the main frame. The ability of their tracks to incline and decline relative to their front skis is also limited, reducing the compliance of their tracks to varying terrain and limiting traction. While the snow cycle described in the '146 patent exhibits improved suspension travel and traction over the earlier snow cycles, it does so by using a track carriage that extends rearward from the snow cycle, making the snow cycle longer than the snow cycles referenced in the '006 and '913 patents. Positioning the track so far rearward from the snow cycle center of gravity limits the braking effectiveness of the track for slowing and stopping the snow cycle.
Human powered snow cycles or "snow bicycles" are also banking snow vehicles. In snow bicycles, a single ski in front and an endless-loop track in back replace the bicycle wheels. Drive power from the operator's legs is transferred from a pedal crank through a chain and sprocket drive train to the track. Snow bicycles are described in U.S. Pat. Nos. 5,423,559 and 5,102,153. The vehicles described in these patents exhibit all of the same limitations as conventional motor driven snow cycles. In addition, the absence of shock absorbers or some other type of suspension between the tracks and the frames of these snow bicycles limits traction and results in an uncomfortable ride over rough terrain.